I am not 100% certain with regards to Lion, though, I did have Trisquel GNU/Linux installed side-by-side with Snow Leopard without refit. It is all a matter of GRUB, if the version of GRUB accompanying the Distribution of your choice is compiled with support for EFI, it should theoretically install side-by-side with Lion. The process would be identical to installing GNU/Linux side-by-side with any other operating system.
- Boot GNU/Linux Live Distro via CD (I believe USB boot is not possible under the MBP)
- Follow the Installer; when your asked about partitions you will have the option to install side-by-side, usually with a graphical slider of some sort.
You may also wish to ask about the installation process for BackTrack 5 with regards to the MacBook Pro on the BackTrack forums themselves; I have witnessed working examples of BT5 RC1 on a MBP, sans ReFIT.